The number of air conditioning apparatuses is rapidly increasing. Having regard to the world wide aim of reduction of carbon dioxide emission, a reduction of the energy consumption of such machines is of utmost importance.
Normally, the energy consumption for air conditioning is largest, when the sunshine is strong. Therefore, using solar energy for conversion to cooling of air is desirable. Especially, it is desirable to use common solar heating systems using water as working fluid.
A system proposed for using waste energy for air conditioning is disclosed in U.S. Pat. No. 6,581,384. This system applies liquids that have a low critical pressure and temperature, such as refrigerants. Though this system seems promising at first glance and the disclosure proposes use of solar heating, a more thorough analysis reveals that this system is not suitable for common solar heating systems, especially not if water is used as working fluid. This will be investigated in greater detail in the following.
FIG. 1 is a copy of the system disclosed in U.S. Pat. No. 6,581,384. A heat source 1, such as a solar heater, provides thermal energy that is transferred to a working fluid in a heat exchanger 2. The working fluid is provided in tube 3 under pressure provided by a liquid pump 4. By receiving thermal energy in the heat exchanger 2, the pressurized liquid is superheated. The superheated liquid is led by tube 5 and control valve 6 into an expander 7, where the liquid is expanded and transfers work from the working fluid into the expander. The working fluid is still superheated after leaving the expander 7 and part of the energy is in heat exchanger 8 transferred from the exit fluid of expander 7 to the working fluid in tube 3. Having received energy from the working fluid, the expander is driving a compressor 9 connected to the expander by a shaft 10. The compressor compresses the working fluid from a gaseous state to an intermediate pressure gas as part of a typical refrigeration cycle. The output fluid from the compressor 9 flows through tube 20 and is co-mingled at branch 11 with the outlet fluid from the heat exchanger 8. In order to extract more heat, a further heat exchanger 12 is used for energy transfer to the working fluid in tube 3. The remaining heat is to a large degree removed by ambient ventilated air cooling in condenser 13. The working fluid exits condenser 13 and is split, where part of the liquid passes through conduit 14 to refrigerant tank 15, where any residual vapour is separated from the liquid before entering the conduit 16 to the pressure pump 4. The other part of the working fluid after splitting follows conduit 17 to an evaporator 18, where the evaporation leads to a lowering of the temperature for uptake of energy from air 19 blown into a building at lowered temperature for air conditioning. The fluid from evaporator 18 is recirculated into the compressor 9.
In U.S. Pat. No. 6,581,384, it is disclosed that the expender receives working fluid of type R134a at a temperature of 400° F., corresponding to 204° C., in order to achieve a superheated fluid, which is illustrated in the reproduced cooling cycle in FIG. 2 being an enthalpy H versus pressure (log P) diagram. The cooling cycle A-B-C-D illustrates expansion A-B in the expander 7, condensing B-C in the condenser 13, pumping C-D in pump 4, and evaporation D-A in heat exchanger 2. Also shown is the cooling cycle for the other part of the split working fluid with evaporation F-E in evaporator 18.
The high temperature of 204° C. of the working fluid is necessary at the expander 7 entrance in order to achieve a superheated fluid with a gaseous phase in the expander 7. The argument for the superheating is not clear from the disclosure but may be due to the fact that the inventor wants to guarantee that no droplets are formed under expansion in the expander, because this would damage the expander blades, as the expander drives at very high speed. The other advantage with a higher temperature on the working fluid is a higher COP value because of the exergy-effect. The Exergy-effect is defined as the energy transformation from a high temperature scale to a lower temperature scale.
The high temperature of 204° C. of the working fluid prevents the use of normal, commercial solar heaters, as these typically work at 70-120° C. and are not designed for such high temperatures of a working fluid.
Having regard to FIG. 2, the movement of point A in the diagram to, for example 140° C., would be an alternative, which is not mentioned in the disclosure of U.S. Pat. No. 6,581,384. However, also this would not be suitable for solar heaters due to a too high temperature.
Another critical point in U.S. Pat. No. 6,581,384 is the mechanical connection in the form of a shaft between the expander and the compressor, where a pressure difference between the expander outlet and the compressor inlet has to be taken into account. This requires a highly sophisticated mechanics, in as much as the shaft is driving at a very high speed. Solution according to prior art will normally reduce the efficiency of the machine.
The system of U.S. Pat. No. 6,581,384 is also not suited for water as a working fluid. This can be easily understood from FIG. 3. No matter the entrance pressure in the heat exchanger 2, the temperature from the solar heater would end at around 100° C. The expansion in the expander from A to B would then happen in the wet area, where liquid is not in the gaseous form, which would damage the expander. Alternatively, the apparatus of U.S. Pat. No. 6,581,384 could start at very low pressure at point A′ or A″ at 100° C., but this would not yield an optimized cooling performance in the end. As a conclusion, the system according to U.S. Pat. No. 6,581,384 is not suited for solar systems with water as a working fluid.